ALBERT

Description: In the past decade many kinematic groups of young stars (〈100 Myr) were discovered in the solar neighbourhood. Since the most interesting period of planet formation overlaps with the age of these groups, their well dated members are attractive targets for exoplanet searches by direct imaging. We combined astrometric, photometric and X-ray data, and applied strict selection criteria to explore the stellar content of five nearby moving groups. We identified more than 100 potential new candidate members in the β Pic moving group, and in the Tucana-Horologium, Columba, Carina and Argus associations. In order to further assess and confirm their membership status, we analysed radial velocity data and lithium equivalent widths extracted from high-resolution spectra of 54 candidate stars. We identified 35 new probable/possible young moving group members: four in the β Pic moving group, 11 in the Columba association, 16 in the Carina association and four in the Argus association. We found serendipitously a new AB Dor moving group member as well. For four Columba systems Hipparcos -based parallaxes have already been available and as they are consistent with the predicted kinematic parallaxes, they can be considered as secure new members.

Description: We present the kinematic results from our ARGOS spectroscopic survey of the Galactic bulge of the Milky Way. Our aim is to understand the formation of the Galactic bulge. We examine the kinematics of about 17 400 stars in the bulge located within 3.5 kpc of the Galactic Centre, identified from the 28 000 star ARGOS survey. We aim to determine if the formation of the bulge has been internally driven from disc instabilities as suggested by its boxy shape, or if mergers have played a significant role as expected from lambda cold dark matter simulations. From our velocity measurements across latitudes b  = –5°, – 7 ${^{\circ}_{.}}$ 5 and –10° we find the bulge to be a cylindrically rotating system that transitions smoothly out into the disc. From observations of 3 fields at b  = +10, the kinematics of the bulge show North-South symmetry about the major axis. Within the bulge, we find a kinematically distinct metal-poor population ([Fe/H] 〈 –1.0) that is not rotating cylindrically. The 5 per cent of our stars with [Fe/H] 〈 –1.0 are a slowly rotating spheroidal population, which we believe are stars of the metal-weak thick disc and halo which presently lie in the inner Galaxy. The kinematics of the two bulge components that we identified in ARGOS Paper III (mean [Fe/H]  –0.25 and [Fe/H]  +0.15, respectively) demonstrate that they are likely to share a common formation origin and are distinct from the more metal-poor populations of the thick disc and halo which are co-located inside the bulge. We do not exclude an underlying merger generated bulge component but our results favour bulge formation from instabilities in the early thin disc.

Description: We present the metallicity results from the ARGOS spectroscopic survey of the Galactic bulge. Our aim is to understand the formation of the Galactic bulge: did it form via mergers, as expected from cold dark matter theory, or from disc instabilities, as suggested by its boxy/peanut shape, or both? Our stars are mostly red clump giants, which have a well-defined absolute magnitude from which distances can be determined. We have obtained spectra for 28 000 stars at a spectral resolution of R  = 11 000. From these spectra, we have determined stellar parameters and distances to an accuracy of 〈1.5 kpc. The stars in the inner Galaxy span a large range in [Fe/H], –2.8 ≤ [Fe/H] ≤ +0.6. From the spatial distribution of the red clump stars as a function of [Fe/H], we propose that the stars with [Fe/H] 〉 –0.5 are part of the boxy/peanut bar/bulge. We associate the lower metallicity stars ([Fe/H] 〈 –0.5) with the thick disc, which may be puffed up in the inner region, and with the inner regions of the metal-weak thick disc and inner halo. For the bulge stars with [Fe/H] 〉 –0.5, we find two discrete populations: (i) stars with [Fe/H]  –0.25 which provide a roughly constant fraction of the stars in the latitude interval b  = –5° to –10°, and (ii) a kinematically colder, more metal-rich population with mean [Fe/H]  +0.15 which is more prominent closer to the plane. The changing ratio of these components with latitude appears as a vertical abundance gradient of the bulge. We attribute both of these bulge components to instability-driven bar/bulge formation from the thin disc. We associate the thicker component with the stars of the early less metal-rich thin disc, and associate the more metal-rich population concentrated to the plane with the colder more metal-rich stars of the early thin disc, similar to the colder and younger more metal-rich stars seen in the thin disc in the solar neighbourhood today. We do not exclude a weak underlying classical merger-generated bulge component, but see no obvious kinematic association of any of our bulge stars with such a classical bulge component. The clear spatial and kinematic separation of the two bulge populations (i) and (ii) makes it unlikely that any significant merger event could have affected the inner regions of the Galaxy since the time when the bulge-forming instabilities occurred.

Description: In this paper, we present an analysis of K2 mission Campaign 3 observations of the irregular Neptune satellite, Nereid. We determined a rotation period of P  = 11.594±0.017 h and amplitude of m  = 0 $.\!\!\!^{{\mathrm{m}}}$ 0328±0 $.\!\!\!^{{\mathrm{m}}}$ 0018, confirming previous short rotation periods obtained in ground-based observations. The similarities of light-curve amplitudes between 2001 and 2015 show that Nereid is in a low-amplitude rotation state nowadays and it could have been in a high-amplitude rotation state in the mid-1960s. Another high-amplitude period is expected in about 30 yr. Based on the light-curve amplitudes observed in the last 15 yr, we could constrain the shape of Nereid and obtained a maximum a : c axis ratio of 1.3:1. This excludes the previously suggested very elongated shape of a : c   1.9:1 and clearly shows that Nereid's spin axis cannot be in forced precession due to tidal forces. Thermal emission data from the Spitzer Space Telescope and the Herschel Space Observatory indicate that Nereid's shape is actually close to the a : c axis ratio limit of 1.3:1 we obtained, and it has a very rough, highly cratered surface.

Description: We present the analysis of photometric and spectroscopic data of two classical Cepheids, FN Aquilae and V1344 Aquilae. Based on the joint treatment of the new and earlier radial velocity data, both Galactic Cepheids have been found to be a member in a spectroscopic binary system. To match the phases of the earlier radial velocity data correctly with the new ones, we also determined the temporal behaviour of the pulsation period of these Cepheids based on all available photometric data. The O  –  C graph covering about half century shows slight changes in the pulsation period due to stellar evolution for both Cepheids.

Description: We present the analysis of photometric and spectroscopic data of six bright Galactic Cepheids: GH Carinae, V419 Centauri, V898 Centauri, AD Puppis, AY Sagittarii and ST Velorum. Based on new radial velocity data (in some cases supplemented with earlier data available in the literature), these Cepheids have been found to be members in spectroscopic binary systems. V898 Cen turned out to have one of the largest orbital radial velocity amplitude (〉40 km s –1 ) among the known binary Cepheids. The data are insufficient to determine the orbital periods nor other orbital elements for these new spectroscopic binaries. These discoveries corroborate the statement on the high frequency of occurrence of binaries among the classical Cepheids, a fact to be taken into account when calibrating the period–luminosity relationship for Cepheids. We have also compiled all available photometric data that revealed that the pulsation period of AD Pup, the longest period Cepheid in this sample, is continuously increasing with P  = 0.004567d century –1 , likely to be caused by stellar evolution. The wave-like pattern superimposed on the parabolic O  – C graph of AD Pup may well be caused by the light-time effect in the binary system. ST Vel also pulsates with a continuously increasing period. The other four Cepheids are characterized with stable pulsation periods in the last half century.

Description: M giants are among the longest period pulsating stars which is why their studies were traditionally restricted to analyses of low-precision visual observations, and, more recently, accurate ground-based data. Here we present an overview of M giant variability on a wide range of time-scales (hours to years), based on the analysis of 13 quarters of Kepler long-cadence observations (one point per every 29.4 min), with a total time-span of over 1000 d. About two-thirds of the sample stars have been selected from the All Sky Automated Survey (ASAS)-North survey of the Kepler field, with the rest supplemented from a randomly chosen M giant control sample. We first describe the correction of the light curves from different quarters, which was found to be essential. We use Fourier analysis to calculate multiple frequencies for all stars in the sample. Over 50 stars show a relatively strong signal with a period equal to the Kepler-year and a characteristic phase dependence across the whole field of view. We interpret this as a so far unidentified systematic effect in the Kepler data. We discuss the presence of regular patterns in the distribution of multiple periodicities and amplitudes. In the period–amplitude plane we find that it is possible to distinguish between solar-like oscillations and larger amplitude pulsations which are characteristic for Mira/semiregular stars. This may indicate the region of the transition between two types of oscillations as we move upward along the giant branch.

Description: We present an analysis of spectroscopic radial velocity and photometric data of three bright Galactic Cepheids: LR Trianguli Australis (LR TrA), RZ Velorum (RZ Vel) and BG Velorum (BG Vel). Based on new radial velocity data, these Cepheids have been found to be members of spectroscopic binary systems. The ratio of the peak-to-peak radial velocity amplitude to photometric amplitude indicates the presence of a companion for LR TrA and BG Vel. IUE spectra indicate that the companions of RZ Vel and BG Vel cannot be hot stars. The analysis of all available photometric data revealed that the pulsation period of RZ Vel and BG Vel varies monotonically, due to stellar evolution. Moreover, the longest period Cepheid in this sample, RZ Vel, shows period fluctuations superimposed on the monotonic period increase. The light-time effect interpretation of the observed pattern needs long-term photometric monitoring of this Cepheid. The pulsation period of LR TrA has remained constant since the discovery of its brightness variation. Using statistical data, it is also shown that a large number of spectroscopic binaries still remain to be discovered among bright classical Cepheids.

Description: Kepler-13b is a most intriguing exoplanet system due to the rapid precession rate, exhibiting several exotic phenomena. We analysed Kepler short cadence data up to Quarter 14, with a total time-span of 928 d, to reveal changes in transit duration, depth, asymmetry and identify the possible signals of stellar rotation and low-level activity. We investigated long-term variations of transit light curves testing for duration, peak depth and asymmetry. We also performed cluster analysis on Kepler quarters. We computed the autocorrelation function of the out-of-transit light variations. Transit duration, peak depth and asymmetry evolve slowly, due to the slowly drifting transit path through the stellar disc. The detected transit shapes will map the stellar surface on the time-scale of decades. We found a very significant clustering pattern with 3-orbit period. Its source is very probably the rotating stellar surface, in the 5:3 spin–orbit resonance reported in a previous study. The autocorrelation function of the out-of-transit light variations, filtered to 25.4 h and harmonics, shows slow variations and a peak around 300–360 d period, which could be related to the activity cycle of the host star.